The present invention relates to semiconductor manufacturing and, more particularly, to the formation of gate oxides and preventing or minimizing gate oxide thinning in gate oxides.
An integrated circuit (IC) consists of thousands of transistors such as metal oxide semiconductors (MOSs). A field oxide (FOX) is disposed between two transistors for insulating the transistors in an IC to prevent short circuit. A gate oxide usually underlies the gate of a transistor and the transistor induces charge through the gate oxide. In the conventional method for forming gate oxide, a FOX is formed on a semiconductor substrate by using local oxidation of silicon (LOCOS) and then a well drive-in process is applied. Because the FOX is in ambient nitrogen at high temperatures during the well drive-in process, a residual nitride such as Si3N4 is formed at a partial edge of the FOX. The residual nitride results in gate oxide thinning at the partial edge of the FOX during formation of the gate oxide.
FIGS. 1A to 1E show a known method for forming a gate oxide on the semiconductor device in which gate oxide thinning occurs. In FIG. 1A, a pad oxide layer 12 and a silicon nitride layer (Si3N4) 14 are formed on a semiconductor substrate 10, respectively.
In FIG. 1B, lithography and etching form the patterned silicon nitride layer 14a and patterned pad oxide layer 12a to expose the surfaces of the substrate 10. Then, the patterned silicon nitride layer 14a is used as a hard mask to etch the surfaces to form trenches 11 in the substrate 10.
In FIG. 1C, the FOX (field oxide) layers 16 are formed in the trenches 11 by wet oxidation. Then, the patterned silicon nitride layer 14a and patterned pad oxide layer 12a are removed to expose the surface of the substrate 10. The exposed region serves as an active area (AA) 10a of the semiconductor device. Subsequently, the ion drive-in process is applied to the active area 10a by injecting nitrogen gas at a high temperature.
In FIG. 1D, a sacrificial oxide layer 18 such as silicon dioxide(SiO2) is formed on the AA 10a. Prior to forming the sacrificial oxide layer 18, silicon nitride (not shown) is formed on the AA 10a and the partial edges of the FOX layers 16 (i.e., the regions xe2x80x9cAxe2x80x9d shown in FIG. 1D) due to exposure of the AA 10a to the nitrogen gas at high temperatures for an extended period of time.
In FIG. 1E, the sacrificial oxide layer 18 is removed, thereby removing the silicon nitride on the AA 10a. Subsequently, an oxide layer 19 is formed on the AA 10a to serve as the gate oxide layer of the semiconductor device by dry oxidation. Only the silicon nitride formed on AA 10a is removed with the sacrificial oxide layer 18. The silicon nitride formed in the partial edge region xe2x80x9cAxe2x80x9d as seen in FIG. 1D often cannot be removed during the removal of the sacrificial oxide layer 18, but remains as the residual silicon nitride prior to formation of the gate oxide. The oxidation rate of the FOX layers 16 near the residual silicon nitride (i.e., the regions xe2x80x9cAxe2x80x9d in FIG. 1D) is lower than that of the surface of the AA 10a when the gate oxide layer 19 is formed. As a result, the gate oxide layer 19 that is formed at the juncture between the FOX layers 16 and the AA 10a is thinner, as shown in regions xe2x80x9cBxe2x80x9d in FIG. 1E. The thinner gate oxide layer 19 causes the increment of the leakage current and the reduction of the breakdown voltage of the semiconductor device.
Embodiments of the present invention relate to a method for preventing gate oxide thinning in a recess LOCOS process. This is achieved by introducing a mixture of oxygen gas and nitrogen gas, typically at an elevated temperature, into the region in which the substrate is disposed during the ion drive-in to the active area of the semiconductor substrate. The affinity for oxygen with silicon is higher than that for nitrogen with silicon. As a result, oxygen can be used to prevent or reduce silicon nitride formation on the field oxide regions. The amount of oxygen is selected to be sufficient to at least substantially prevent silicon nitride from forming in the field oxide regions, thereby preventing or reducing gate oxide thinning during the subsequent formation of the gate oxide due to the presence of residual silicon nitride. In specific embodiments, an upper oxide layer is formed on the active area during the ion drive-in to the active region. The upper silicon oxide layer 28 can prevent an excess of nitrogen bonds with silicon from forming in the active area to produce a lot of silicon nitride (Si3N4), which can be difficult to remove before forming the gate oxide layer.
In accordance with an aspect of the present invention, a method of forming a gate oxide on a substrate comprises providing a substrate having thereon a plurality of trenches separated by a patterned pad oxide and a patterned silicon nitride layer disposed thereon and used to form the plurality of trenches. A plurality of first oxide layers are formed in the trenches as field oxides of a semiconductor device. The patterned silicon nitride layer and the patterned pad oxide layer are removed to expose a surface of the substrate as an active area of the semiconductor device. A flow of oxygen and nitrogen with a predetermined flow ratio is introduced for performing an ion drive-in to the active area at a predetermined temperature. The method further comprises forming a second oxide layer on the active area, removing the second oxide layer to expose the active area, and forming a third oxide layer on the active area as a gate oxide of the semiconductor device.
In some embodiments, the predetermined flow ratio of the oxygen to the nitrogen is in a range of about 0.5% to about 2%. The fourth oxide layer is formed by a dry oxidation process. The thickness of the fourth oxide layer is in a range of 100 xc3x85 to 200 xc3x85. The first oxide layers are formed by a wet oxidation process. The predetermined temperature for the ion drive-in is in a range of about 800xc2x0 C. to about 1200xc2x0 C. The second oxide layer is a sacrificial oxide layer comprising silicon dioxide. The third oxide layer is formed by a dry oxidation process. A fourth oxide layer is formed on the active area during the ion drive-in to the active area. The trenches are formed by using an etching process.
Another aspect of the present invention is directed to a method of preventing or reducing gate oxide thinning in a recess LOCOS process for a substrate including thereon an active area and field oxide regions disposed adjacent the active area. The method comprises performing an ion drive-in to the active area on the substrate by directing a flow of oxygen and nitrogen toward the substrate at a predetermined temperature and with a sufficient amount of oxygen to at least substantially prevent silicon nitride from forming on the field oxide regions. The method further comprises forming a sacrificial oxide layer on the active area, removing the sacrificial oxide layer to expose the active area, and forming a gate oxide layer on the active area.
In some embodiments, the flow of oxygen and nitrogen comprises O2 and N2 at a ratio of about 0.5% to about 2% O2/N2. During the ion drive-in to the active area, a residual nitride is formed on the active area and an upper oxide layer is formed over the residual nitride. The upper oxide layer has a thickness of about 100 xc3x85 to about 200 xc3x85. The residual nitride and the upper oxide layer are removed during removal of the sacrificial oxide layer.
In accordance with another aspect of the invention, a method of forming a gate oxide on a substrate comprises providing a substrate having thereon a plurality of trenches having gate oxides formed therein, wherein the plurality of trenches are separated by a patterned pad oxide and a patterned silicon nitride layer disposed thereon and used to form the plurality of trenches. The patterned silicon nitride layer and the patterned pad oxide layer are removed to expose a surface of the substrate as an active area of the semiconductor device. An ion drive-in to the active area on the substrate is performed by directing a flow of oxygen and nitrogen toward the substrate at a predetermined temperature and with a sufficient amount of oxygen to at least substantially prevent silicon nitride from forming on the field oxide regions. The method further comprises forming a sacrificial oxide layer on the active area, removing the sacrificial oxide layer to expose the active area, and forming a gate oxide layer on the active area.